^"^'"'I ZOOGENESIS ^1^ 



greater than the structural range in certain single 

 species of insects or of crustaceans at different stages 

 in their life history. The vertebrates possess such a 

 delicately balanced complexity of internal structure 

 and, partly as a result of their large size, such a deli- 

 cate adjustment to their environment, that changes 

 brought about by a continuous series of slight altera- 

 tions and progressive minor readjustments are more 

 suited to them than the sudden wide and abrupt dis- 

 continuities so frequent in invertebrate types. 



Because of their very high degree of specialization, 

 well marked and reasonably continuous evolutionary 

 lines are frequent among the vertebrates, and wide 

 discontinuities are relatively rare, while the reverse 

 is true in all the other phyla of comparable size. 



That the gaps between the forms in individual 

 species and the wide discontinuities between the 

 several classes included in each of the larger phyla 

 are fundamentally of the same nature and differ only 

 in degree can scarcely be denied. The latter are to be 

 interpreted as having resulted originally from muta- 

 tions in creatures which were of a primitive nature and 

 therefore capable of producing young very widely dif- 

 ferent from themselves yet able to meet the require- 

 ments of existence. 



The relationship of man. — Since he possesses a back- 

 bone and associated structures, man belongs to the 

 group of backboned animals or vertebrates, which 

 group includes the fishes, amphibians, reptiles, birds 

 and mammals . Within the vertebrates he is a member 

 of the group of mammals. Within the mammals he 



[2.2.3] 



